[S-IT19] 地球深部科学

[SIT19-04] キンバーライトの初生W同位体組成推定

小林 旺太郎¹、町田 嗣樹²、永治 方敬¹、兼岡 一郎¹、*飯塚 毅¹ (1.東京大学、2.千葉工業大学)

キーワード：キンバーライト、タングステン同位体、核ーマントル相互作用

Kimberlites are considered to be geochemically important rocks due to their unique isotopic compositions. Recent studies have reported whole-rock ¹⁸²W/¹⁸⁴W data of kimberlites to constrain the characteristics of their source mantle (Tappe et al., 2020; Nakanishi et al., 2021). However, because kimberlites are susceptible to secondary alteration and crustal contamination, analyses using whole-rock samples are unlikely to provide their original isotopic signatures. In this study, we applied the acid leaching and magnetic separation techniques to kimberlites from China, South Africa, and Brazil to separate the primitive values from secondary modified ones. Among the samples analyzed in this study, the residue fraction of Chinese kimberlite showed μ¹⁸²W value of -12.2 ± 4.3 (2SE) and this value is distinctly different from the value of its leachate fraction, which lacked anomalous ¹⁸²W signatures. The former value is considered to represent primitive W isotopic compositions of kimberlite source, whereas the latter value is considered to reflect the contribution of secondary alteration and crustal contamination. This result could reconcile the discrepancy between the results reported by Tappe et al. (2020) and Nakanishi et al. (2021). The negative μ¹⁸²W anomaly in the kimberlite source mantle can be attributed to either 1) contribution of early-differentiated silicate reservoir, 2) an excess of late-accreted materials, or 3) core-mantle interaction. All of these processes support the possibility that kimberlites originated from a lower mantle region.